Measuring concentration ("solids loading") and velocity ("flow rate") in dynamic gas-solid or liquid-solid ("multi-phase") systems has traditionally been a complicated problem. Standard techniques do not provide enough information and are often plagued by low sensitivity. Moreover, few are flexible enough to be employed in an in-situ environment.
Velocities of multi-phase streams, such as those including suspended solids, have been measured by cross correlation of upstream and downstream detector outputs based on the time change required for suspended solids to flow from the upstream detector to the downstream detector. Procedures of this type may not be of adequate dependability if, for example, disturbances occur in the multi-phase stream that scramble the position or orientation of the suspended solids being tracked during transit thereof between the two detectors. Such scrambling may occur, particularly, in turbulent stream flow.
Where two or more spaced probes are used, alignment of the probes with respect to the direction of stream flow is essential. Additionally, use of multiple spaced probes inherently extends the time in which a desired analysis of a multi-phase stream flow can be accomplished.
In an effort to stabilize stream flow to permit a more accurate determination of velocity, a system utilizing a vortex-producing member located upstream from a pair of spaced probes has been proposed. A series of time-related vortices is generated and autocorrelations of the outputs of the spaced probes, based on the time period between consecutively generated vortices, are averaged.
Another approach to determining the flow rate of a multi-phase stream involves fiber optic illumination of at least a segment of the stream and transmission of images of particles flowing past an endoscope as recorded with a camera and viewed on a monitor. Under this method, image analysis is relied upon to obtain flow rate.
Other known systems use optical fibers to measure bubble flow rate and size. One such system relies on Doppler shifting of monochromatic light. In operating this system, a bubble impinges on one end of an optical fiber with the fiber penetrating a first surface portion of the bubble. Subsequently, upon movement of the bubble, a second surface portion is penetrated by the fiber in order to obtain a velocity measurement.
As has been explained, most known systems rely on cross correlation techniques. Alignment of the sensors is critical if such techniques are to be effective. Additionally, two channels of data need to be sensed and processed. An advancement over these known flow rate determination procedures is disclosed by Lyons et al. (U.S. Pat. No. 4,978,863). Lyons et al. disclose a method for determining the flow rate of particles in a fluid medium by measuring backscattered light, converting the backscattered light to voltage, digitizing the voltage at discrete points in time to form an array of numbers ("voltage waveform"), and autocorrelating this voltage waveform. The autocorrelation of more than one voltage waveform at different time delays may then be normalized by the value of the autocorrelation at zero time. Lyons et al. disclose that each time delay corresponding to fifty percent decorrelation is inversely proportional to the flow rate of the stream. Lyons et al. use an apparatus that comprises: a single fiber optic probe provided with at least two optical fibers, one to transmit light and the other to collect backscattered light; a light source; a light detector; and an autocorrelator. The single probe design eliminates the criticality of detector alignment demonstrated in prior art.
Notwithstanding the advances by Lyons et al., some problems still persist. First, if the concentration fluctuates significantly during the voltage waveform acquisition, a time constant for these fluctuations will be reflected in the fifty percent autocorrelation time since autocorrelation is essentially a time averaging function. Second, extensive array manipulations inherent in an autocorrelation algorithm require a great deal of computing time. For example, a single analysis can run over twelve minutes on an 8 mHz computer. Third, when several particles simultaneously occupy a field of view and each particle is only slightly resolved, the autocorrelation technique treats multiple response peaks (one for each particle) as a single peak whose width will be averaged with that of other arrays. Thus, the more concentrated the particles in a fluid medium, the less accurate the autocorrelated velocity calculation.
In addition, at present, no single technique is both flexible and practical enough to simultaneously determine the concentration and velocity of multiple particles in concentrated gas-solid media. This shortcoming plagues efforts to model kinetic and hydrodynamic behavior in fluidized beds and riser reactors.